General Discussion

The holothurian fauna of the Cook Strait region is notable for its diversity at the generic level. The Order Dendrochirotida is well represented on the shelf, but only one species, Ocnus calcareus may be found intertidally. In contrast, three of the four apodous species appear to be restricted to the intertidal zone where five holothurian species are known to occur.

They are:

Kolostoneura novae-zealandiae	Trochodota dunedinensis
Trochodota dendyi	Ocnus calcareus
Stichopus mollis

Of the above species, Trochodota dendyi and Ocnus calcareus are relatively rare, while the remaining three species are common.

The following eight species have been taken from the shelf:

Protankyra uncinata	Neocucumella bicolumnata
Neothyonidium dearmatum	Pentadactyla longidentis
Heterothyone alba	Heterothyone ocnoides
Stolus huttoni	Stichopus mollis

All but three of these species are restricted to the shelf. Pentadactyla longidentis has been found at 400 fathoms (Station 87), and Heterothyone alba and Stichopus mollis are known from 100—150 fathoms (Station 15). It is possible that specimens taken from these depths may have strayed accidentally from the shelf into deep water, as a result of the steep marine profiles in the area. Unfortunately it is not known whether the specimens were alive when captured. Probably most of the specimens known so far only from Wellington Harbour occur also on the Cook Strait shelf, but there is a definite need for a more thorough investigation of the shelf and slope.

There are three deep water species known from the area. They are:

Heteromolpadia marenzelleri	Molpadia violacea
Paracaudina chilensis

As a result of the greater depth tolerances in the above species, they should have the ability to achieve a wide distribution by spreading across the deep sea floor. Although Heteromolpadia marenzelleri is known so far only from New Zealand, Molpadia violacea also occurs near Kerguelen (Theel, 1886), and Paracaudina chilensis is a circum-Pacific species. It is expected that further deep water investigation in Cook Strait will reveal members of the almost exclusively deep sea group, Order Elasipodida.

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Present records indicate that no shelf holothurian species are restricted to the Cookian Province as defined by Finlay (1925) and applied to the echinoderms by Fell (1949) and Pawson (1961). The fauna of Cook Strait contains a mixture of northern and southern New Zealand elements, with southern elements predominating. Cook Strait does not appear to present a barrier to northward dispersal of southern species.

Among the Cook Strait holothurians are four species which are also known in Australia. They are:

Paracaudina chilensis	Neocucumella bicolumnata
Neothyonidium dearmatum	Stichopus mollis

The above species comprise about 25% of the known fauna. The means by which echinoderms can cross the ocean gap between Australia and New Zealand have been discussed by Mortensen (1925) and Fell (1953). Drift in surface waters across the Tasman Sea in the planktonic larval stage is feasible for those species which possess pelagic larvae (as is probably the case with Stichopus mollis). Both Neothyonidium dearmatum and Neocucumella bicolumnata have yolky eggs, and probably lack a larval stage in their life history. They may have been carried across the Tasman Sea in rafts of seaweed, or perhaps dispersed via the Lord Howe Rise to which Fell (1953) attaches some importance as at least a former, if not a present, dispersal route. More complete information on the life history and bathymetric distribution of such problematical species may assist in revealing their dispersal mechanism.